The skin provides a protective barrier against foreign materials and infection. In mammals this is accomplished by forming a highly insoluble protein and lipid structure on the surface of the corneocytes termed the cornified envelope (CE). (Downing et al., Dermatology in General Medicine, Fitzpatrick, et al., eds., pp. 210-221 (1993), Ponec, M., The Keratinocyte Handbook, Leigh, et al., eds., pp. 351-363 (1994)). The CE is composed of polar lipids, such as ceramides, sterols, and fatty acids, and a complicated network of cross-linked proteins; however, the cytoplasm of stratum corneum cells remains polar and aqueous. The CE is extremely thin (10 microns) but provides a substantial barrier. Because of the accessibility and large area of the skin, it has long been considered a promising route for the administration of drugs, whether dermal, regional, or systemic effects are desired.
A topical route of drug administration is sometimes desirable because the risks and inconvenience of parenteral treatment can be avoided; the variable absorption and metabolism associated with oral treatment can be circumvented; drug administration can be continuous, thereby permitting the use of pharmacologically active agents with short biological half-lives; the gastrointestinal irritation associated with many compounds can be avoided; and cutaneous manifestations of diseases can be treated more effectively than by systemic approaches.
Most transdermal delivery systems achieve epidermal penetration by using a skin penetration enhancing vehicle. Such compounds or mixtures of compounds are known in the art as “penetration enhancers” or “skin enhancers”. While many of the skin enhancers in the literature enhance transdermal absorption, several possess certain drawbacks in that (i) some are regarded as toxic; (ii) some irritate the skin; (iii) some have a thinning effect on the skin after prolonged use; (iv) some change the intactness of the skin structure resulting in a change in the diffusability of the drug; and (v) all are incapable of delivering high molecular weight pharmaceuticals and cosmetic agents. Clearly there remains a need for safe and effective transdermal delivery systems that can administer a wide-range of pharmaceuticals and cosmetic agents.